In the past, there has been provided a communication system which establishes encrypted communication using a session key having an available period. In this communication system, the session key is encrypted and then distributed. The session key is encrypted by use of common key cryptography which is referred to as KryptoKnight protocol, for example.
According to the common key cryptography, the same common key (hereinafter referred to as “communication secret key”) is preliminarily given to the server (e.g., a residential server) and the terminal (e.g., an equipment device). Each terminal stores its own communication secret key, which remains unchanged even if the terminal becomes available by another user. Consequently, if a previous user of the terminal retrieves the communication secret key from the same terminal, the previous user can eavesdrop on communication between the server and the same terminal. Therefore, the previous user can monitor a usage situation (e.g., a lifestyle) of a current user of the same terminal, and can use the same terminal fraudulently (operate the same terminal without permission from the current user).
Document 1 (JP 2000-349748) discloses public key cryptography which offers higher security than the common key cryptography. Even if the public key cryptography is used, it is difficult to completely prevent the previous user from eavesdropping communication unless a public key and a private key are changed.
It has been proposed to change a public key and a public key in response to a change of a user. However, according to the public key cryptography disclosed in the document 1, one of a sender and a receiver has to hold a secret key (private key) paired with a public key of the other in order to share information with each other. Accordingly, the public key cryptography disclosed in the document 1 is unable to exchange the secret key itself.